1. Field of the Invention
This invention relates to semiconductor devices, Micro Electro Mechanical Systems (MEMS), sensors and more specifically to three dimensional (3D) three-axis force input control devices, finger force sensors, finger-mouse, micro-joysticks and their fabrication.
2. Background
The 3D force sensors based on micromachined silicon chips with stress sensitive components on the flexible diaphragm are known. Prior art, described in U.S. patent application Ser. No. 11/025,642, is shown in FIG. 1. It uses a sensor chip 10 with a rigid frame 12 and a rigid central part (or rigid island or boss) 16 of a die, connected with the frame by a thinner elastic diaphragm 14. A rigid force-transferring element 31 is firmly coupled to the rigid island 16 of a sensor die 10 through an intermediate layer 33. When an externally applied force is transferred to the rigid island 16, the deflection of the island creates deformation and stress in the elastic diaphragm element 14, reaching the locations of the stress sensitive IC components 18, 20, 22. Because the rigid island 16 is located within the thickness dimension of the sensor die 10 and the width of the diaphragm is small and in the sub-millimeter range, it is difficult to apply a force to the rigid island 16 without applying it to the frame of the die. As a result, the sensitivity to the external force in lateral X and Y directions is not sufficient to provide good characteristics of 3D force input control device.
In order to resolve this deficiency a rigid force-transferring element 31 is added to the structure of the die 10 and island 16. This rigid force-transferring element 31 is firmly coupled to the rigid island 16 of a sensor die 10 through an intermediate layer 33. This extension of the rigid island height above the die frame thickness enhances sensor sensitivity to all components of the applied vector force. These components of force vector are transferred to the rigid island 16 of the sensor die 10 and then to the elastic element 14 and sensitive IC components 18, 20, 22.
The prior art is describing the device however doesn't teach the methods of fabrication of these devices.
The prior art teaches that by changing the size of the rigid force-transferring element 31 it is possible to change the ratio of sensitivities in X and Y directions, on one hand, and Z direction, on the other hand. However the prior art device provides only limited range of sensitivities ratio and does not provide an opportunity to make X and Y sensitivities also unequal with predetermined ratio. The need for having all three sensitivities unequal can be illustrated by gaming application, where motion of the gaming object in X and Y directions could be different and the action function in Z direction, as a rule, requires higher force and therefore lower sensitivity.
Depending on the specific application the trench 15 between frame 12 and rigid island 16 could be either filled in with elastic plastic material or not independently on the size of the rigid force-transferring element 31. The prior art doesn't teach how to do that.
For most of the high volume applications of three-dimensional (3D) force input control devices the most important requirement is their very low cost. Therefore the least costly and most efficient methods of fabrication are critical for such devices.